1. Field of the Invention
The present invention relates to a solid-polymer-electrolyte membrane for a polymer-electrolyte fuel cell, and a process for producing the same.
2. Description of Related Art
A polymer-electrolyte fuel cell employs hydrogen and oxygen as a fuel and an oxidizing agent, respectively, and is considered a promising small-and-lightweight power source to be applied to automobiles, etc. Such a fuel cell comprises a solid-polymer-electrolyte membrane having opposite surfaces, a positive electrode disposed in contact with one of the opposite surfaces of the membrane, and a negative electrode disposed in contact with another one of the opposite surfaces of the membrane. Hydrogen is oxidized electrochemically at the negative electrode to produce protons and electrons. The protons are transferred through the solid-polymer-electrolyte membrane to the positive electrode, to which oxygen is supplied. Whilst, the electrons produced at the negative electrode flow by way of a load, which is connected to the fuel cell, to the positive electrode. At the positive electrode, the electrons react with protons and oxygen to produce water.
It has been known that the performance of a fuel cell depends greatly on the performance of the gas diffusion electrodes used as the positive and negative electrodes, and on the performance of solid-polymer-electrolyte membrane. The performance required for solid-polymer-electrolyte membrane is to permit as many protons as possible to flow. In order to establish the required performance, it has been known that it is important to introduce as many sulfonic groups as possible which are capable of imparting and ion-exchange capability to solid-polymer-electrolyte membrane.
A fluorocarbon-based resin, for example, Nafion (Trade Mark) and its derivatives, has been known as a few of the representatives of the solid-polymer-electrolyte membranes for the fuel-cell application. Nafion is based on a copolymer made from tetrafluoroethylene and perfluorovinylether, and is provided with sulfonic groups working as ion-exchanging groups.
However, solid-polymer-electrolyte membranes formed of Nafion, and the like, liquify when the sulfonic groups, working as ion-exchanging groups, are introduced into the membrane in an increasing quantity in order to decrease the electric resistance of the membranes. Thus, the sulfonic groups should be introduced into the membranes in a limited quantity. Moreover, as the quantity of the introduced sulfonic groups increases, the strength of the resulting membranes degrades. When the membranes have a low electric resistance, they suffer from a problem in that they break during operation of the fuel cells. Due to these reasons, membranes formed of Nafion, and the like, exhibit an ion-exchanging capacity of 1.1 milli-equivalent/g at the highest. Thus, it has been desired that the membranes be further improved in terms of ion conductivity.
Ion-exchange membranes formed of Nafion, and the like, have been put into practical applications in the field of brine electrolysis industry, and have been known to have a good chemical stability. However, Nafion, and the like, are very expensive, because they are fluorocarbon-based resins. Considering the application of polymer-electrolyte fuel cell to automobiles, it is required that the current cost of solid-polymer-electrolyte membrane formed of Nafion be decreased by about one-to-a couple of dozens to one-to-a couple of hundreds. If not, the fuel cell can hardly be believed to be put into practical applications.
There are other approaches for preparing solid-polymer-electrolyte membranes: namely; side chains, into which sulfonic groups can be introduced, are brought into base films by radiation-graft polymerization. For instance, styrene, or the like, is graft-polymerized into a Teflon (Trade Mark) membrane or a Teflon-based copolymer film, and thereafter sulfonic groups are introduced into the resulting graft-polymerized polystyrene chain. The solid-polymer-electrolyte membranes prepared by this process cannot contribute to attaining sufficient fuel cell performance because of the problems hereinafter described.
As described in Electrochimica, Acta 40,345 (1995), a fuel cell was prepared by using a solid-polymer-electrolyte membrane. The membrane was prepared by graft-polymerizing styrene onto tetrafluoroethylene-hexafluoropropylene (i.e., FEP) copolymer film exposed to gamma radiation. Sulfonic groups were then introduced into the thus graft-polymerized copolymer film to prepare the membrane, and the resulting membrane was incorporated into a fuel cell. The literature reports that, immediately after operating the fuel cell, the membrane was decomposed, and thereby the sulfonic groups were eliminated. As a result, the internal resistance of the fuel cell was increased, and the performance thereof was deteriorated sharply even after operating its fuel cell for couple of dozens of hours. The literature also refers to the fact that the fuel cell lacks sufficient output performance because the membrane was inferior to Nafion in terms of ion conductivity. The literature further reveals that the inadequate chemical stability of the graft-polymerized polystyrene side chains resulted in the decomposition of the membrane under the operating conditions of the fuel cell.
In addition, other processes have been known for preparing polymer ion-exchange membranes, and are premised on the recognition that sulfonated polystyrene side chains have insufficient chemical stability. According to these processes, .alpha., .beta., .beta.-trifluorostyrene, one of fluorinated styrenes, is graft-polymerized into a tetrafluoroethylene polymer membrane or a tetrafluoroethylene-based copolymer membrane, and the graft-polymerized membrane is sulfonated to prepare a polymer ion-exchange membrane. For example, see U.S. Pat. No. 4,012,303 and U.S. Pat. No. 4,605,685. These U.S. patents do not specifically describe the operating characteristics of the resulting polymer ion-exchange membranes which are applied to a polymer-electrolyte fuel cell. The inventors of the present invention, however, estimate that the membranes suffer from the following problems.
Firstly, fluorinated styrene or .alpha., .beta., .beta.-trifluorostyrene pushes up the manufacturing cost, because it is difficult to synthesize fluorinated styrene or .alpha., .beta., .beta.-trifluorostyrene. Thus, similar to the problem associated with Nafion, and the like, the cost problem hinders the polymer ion-exchange membranes from the application to solid-polymer-electrolyte fuel cell.
Secondly, in the radiation-graft polymerization of .alpha., .beta., .beta.-trifluorostyrene, the reactivity is so low that .alpha., .beta., .beta.-trifluorostyrene can be introduced limitedly into the tetrafluoroethylene polymer membrane or tetrafluoroethylene-based copolymer membrane in an amount of 50% by weight or less. As a result, the sulfonic groups cannot be introduced into the graft-polymerized membrane in a large amount. Hence, in the application to solid-polymer-electrolyte fuel cell, the membranes lack sufficient ion conductivity, similar to the membranes formed of Nafion, and the like. Thus, the membranes cannot solve the problems associated with the membranes formed of Nafion, and the like.
Thirdly, the membranes exhibit such a degree of flexibility that they are likely to break during preparation or in the operation of a fuel cell.